Holographic Waveguide Backlight and Related Methods of Manufacturing

ABSTRACT

Systems and methods for holographic waveguide backlights in accordance with various embodiments of the invention are illustrated. One embodiment includes an optical illumination device including at least one waveguide, a source of light optically coupled to the at least one waveguide configured to emit light having a first polarization state, a first plurality of grating elements for diffracting the light having the first polarization state out of the at least one waveguide into a first set of output paths, a second plurality of grating elements for diffracting the light having the first polarization state light out of the at least one waveguide into a second set of output paths, and at least one input coupler configured to couple at least a portion of the light having the first polarization state towards the first and second pluralities of grating elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 17/124,269 entitled “Holographic Waveguide Backlight andRelated Methods of Manufacturing,” filed Dec. 16, 2020, which U.S.patent application Ser. No. 16/817,524 entitled “Holographic WaveguideBacklight and Related Methods of Manufacturing,” filed Mar. 12, 2020,which application claims the benefit of and priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 62/817,468 entitled“Holographic Waveguide Backlight,” filed Mar. 12, 2019, the disclosureswhich are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention generally relates to waveguide devices and, morespecifically, to holographic waveguide backlights.

BACKGROUND

Waveguides can be referred to as structures with the capability ofconfining and guiding waves (i.e., restricting the spatial region inwhich waves can propagate). One subclass includes optical waveguides,which are structures that can guide electromagnetic waves, typicallythose in the visible spectrum. Waveguide structures can be designed tocontrol the propagation path of waves using a number of differentmechanisms. For example, planar waveguides can be designed to utilizediffraction gratings to diffract and couple incident light into thewaveguide structure such that the in-coupled light can proceed to travelwithin the planar structure via total internal reflection (TIR).

Fabrication of waveguides can include the use of material systems thatallow for the recording of holographic optical elements within thewaveguides. One class of such material includes polymer dispersed liquidcrystal (PDLC) mixtures, which are mixtures containingphotopolymerizable monomers and liquid crystals. A further subclass ofsuch mixtures includes holographic polymer dispersed liquid crystal(HPDLC) mixtures. Holographic optical elements, such as volume phasegratings, can be recorded in such a liquid mixture by illuminating thematerial with two mutually coherent laser beams. During the recordingprocess, the monomers polymerize, and the mixture undergoes aphotopolymerization-induced phase separation, creating regions denselypopulated by liquid crystal micro-droplets, interspersed with regions ofclear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating. Theresulting grating, which is commonly referred to as a switchable Bragggrating (SBG), has all the properties normally associated with volume orBragg gratings but with much higher refractive index modulation rangescombined with the ability to electrically tune the grating over acontinuous range of diffraction efficiency (the proportion of incidentlight diffracted into a desired direction). The latter can extend fromnon-diffracting (cleared) to diffracting with close to 100% efficiency.

Waveguide optics, such as those described above, can be considered for arange of display and sensor applications. In many applications,waveguides containing one or more grating layers encoding multipleoptical functions can be realized using various waveguide architecturesand material systems, enabling new innovations in near-eye displays foraugmented reality (AR) and virtual reality (VR), compact head-updisplays (HUDs) and helmet-mounted displays or head-mounted displays(HMDs) for road transport, aviation, and military applications, andsensors for biometric and laser radar (LIDAR) applications.

SUMMARY OF THE INVENTION

Systems and methods for holographic waveguide backlights in accordancewith various embodiments of the invention are illustrated. Oneembodiment includes an optical illumination device including a lightguiding structure with an upper surface for extracting illumination anda lower surface, a light source optically coupled to the light guidingstructure and configured to provide polarized light, the lightundergoing total internal reflection within the light guiding structure,and at least one plurality of grating elements disposed in at least onegrating layer for extracting light from the light guiding structure.

In another embodiment, the light source is configured to emit at leastfirst and second wavelength collimated light color sequentially, whereinthe at least one plurality of grating elements includes a firstplurality of grating elements for diffracting the first wavelength lightout of the light guiding structure into a first set of output paths, anda second plurality grating elements for diffracting the secondwavelength light out of the light guiding structure into a second set ofoutput paths substantially overlapping the first set of output paths.

In a further embodiment, the optical illumination device furtherincludes a substrate having half-wave retarding regions interspersedwith clear regions overlaying the upper surface, wherein each the halfwave retarding region overlaps at least one grating element in each ofthe first and second pluralities of grating elements, and each the clearregion overlaps at least one grating element in each of the first andsecond pluralities of grating elements.

In still another embodiment, the optical illumination device furtherincludes a quarter-wave retarding layer disposed, the quarter-waveretarding layer having a first surface disposed in proximity to thelower surface and a reflective surface.

In a still further embodiment, the first plurality of grating elementsis disposed in a separate grating layer to the second plurality ofgrating elements, wherein grating elements for diffracting the firstwavelength light overlap grating elements for diffracting the secondwavelength light.

In yet another embodiment, grating elements for diffracting first andsecond wavelength light are disposed as uniformly interspersed first andsecond multiplicities of grating elements in one layer.

In a yet further embodiment, grating elements for diffracting first andsecond wavelength light are disposed as uniformly interspersed first andsecond multiplicities of grating elements in two layers, wherein gratingelement for diffracting a first wavelength light overlap gratingelements for diffracting second wavelength light.

In another additional embodiment, grating elements for diffracting firstwavelength light have a first grating vector and grating elements fordiffracting second wavelength light have a second grating vector in anopposing direction to the first grating vector.

In a further additional embodiment, grating elements for diffractingfirst wavelength light and grating elements for diffracting secondwavelength light have grating vectors aligned in substantially paralleldirections.

In another embodiment again, grating elements for diffracting firstwavelength light and grating elements for diffracting second wavelengthlight are off-Bragg with respect to each other.

In a further embodiment again, grating elements for diffracting firstwavelength light are disposed in a first layer in which grating elementshaving a first grating vector and grating elements having a secondgrating vector in an opposing direction to the first grating vector areuniformly interspersed, wherein grating elements for diffracting secondwavelength light are disposed in a second layer in which gratingelements having a first grating vector and grating elements having asecond grating vector in an opposing direction to the first gratingvector are interspersed.

In still yet another embodiment, the first wavelength light has a firstpolarization and the second wavelength light has a second polarizationorthogonal to the first polarization.

In a still yet further embodiment, the first wavelength light and thesecond wavelength light have the same polarization.

In still another additional embodiment, grating elements for diffractingfirst and second wavelength light are disposed as first and secondmultiplicities of grating elements multiplexed in a single layer,wherein grating elements for diffracting the first wavelength aremultiplexed with grating elements for diffracting the second wavelengthlight.

In a still further additional embodiment, grating elements fordiffracting first and second wavelength light are disposed as first andsecond multiplicities of grating elements in a stack of two contactinglayers with grating elements for diffracting the first wavelength lightoverlapping grating elements for diffracting the second wavelengthlight.

In still another embodiment again, grating elements of the firstplurality are switched into a diffracting state when the light sourceemits the first wavelength light and grating elements of the secondplurality are switched into a diffracting state when the light sourceemits the second wavelength light.

In a still further embodiment again, the output paths are angularlyseparated.

In yet another additional embodiment, the output paths are substantiallynormal to the upper surface.

In a yet further additional embodiment, the at least one plurality ofgrating elements is disposed in at least one grating layer, wherein thelight guiding structure includes at least one waveguide, wherein eachthe waveguide supports at least one of the grating layers.

In yet another embodiment again, the layer is formed between transparentsubstrates with transparent conductive coatings applied to each thesubstrate, at least one of the coatings being patterned intoindependently addressable elements overlapping the grating elements,wherein an electrical control circuit operative to apply voltages acrosseach the grating elements is provided.

In a yet further embodiment again, each the grating element includes atleast one property that is one of a planar Bragg surfaces, opticalpower, optical retardation, diffusing properties, spatially varyingdiffraction efficiency, diffraction efficiency proportional to a voltageapplied across the grating element, and phase retardation proportionalto a voltages applied across the grating element.

In another additional embodiment again, the at least one plurality ofgrating elements includes a two-dimensional array.

In a further additional embodiment again, the at least one plurality ofgrating elements includes a one-dimensional array of elongate elements.

In still yet another additional embodiment, each the grating element isrecorded in a Holographic Polymer Dispersed Liquid Crystal.

In a still yet further additional embodiment, the light is coupled intothe light guide structure by a grating or a prism.

In yet another additional embodiment again, the light source is laser orLED.

In a yet further additional embodiment again, the optical illuminationdevice further includes at least one component that is one of a beamdeflector, a dichroic filter, a microlens array, beam shaper, lightintegrator, and a polarization rotator.

A still yet another embodiment again includes an optical illuminationdevice including at least one waveguide, a source of light opticallycoupled to the at least one waveguide configured to emit light having afirst polarization state, a first plurality of grating elements fordiffracting the light having the first polarization state out of the atleast one waveguide into a first set of output paths, a second pluralityof grating elements for diffracting the light having the firstpolarization state light out of the at least one waveguide into a secondset of output paths, and at least one input coupler configured to coupleat least a portion of the light having the first polarization statetowards the first and second pluralities of grating elements.

In a still yet further embodiment again, the optical illumination devicefurther includes a quarter-wave plate having a reflective surface, and asubstrate including a plurality of transparent regions and a pluralityof regions supporting half-wave plates, wherein at least one of thefirst plurality of grating elements is configured to diffract a firstportion of the light having the first polarization state towards atleast one of the plurality of transparent regions, at least one of thesecond plurality of grating elements is configured to diffract a secondportion of the light having the first polarization state towards thequarter-wave plate, and the quarter-wave plate is configured to reflectincident the light having the first polarization state towards at leastone of the plurality of regions supporting half-wave plates, wherein thereflected incident light has its polarization state changed to a secondpolarization state that is orthogonal to the first polarization state,wherein the first and second pluralities of grating elements are formedin at least one grating layer disposed within the at least onewaveguide.

In still yet another additional embodiment again, the opticalillumination device further includes third and fourth pluralities ofgrating elements, wherein the light having a first polarization stateincludes light of a first wavelength band and light of a secondwavelength band, the at least one input coupler includes a first inputcoupler for coupling the light of the first wavelength band towards thefirst and second pluralities of grating elements, and a second inputcoupler for coupling the light of the second wavelength band towards thethird and fourth pluralities of grating elements.

In a still yet further additional embodiment again, the at least onewaveguide includes first and second grating layers, the first and secondpluralities of grating elements are interspersed within the firstgrating layer, the third and fourth pluralities of grating elements areinterspersed with the second grating layer, the first and thirdpluralities of grating elements have grating vectors in a firstdirection, and the second and fourth pluralities of grating elementshave grating vectors in an opposing direction to the first direction.

In still another additional embodiment again, the emitted light iscollimated light, and source of light is configured to emit the light ofthe first and second wavelength bands sequentially.

In a still further additional embodiment again, the first and secondpluralities of grating elements are configured to switch into adiffracting state when the source of light emits the light of the firstwavelength band is emitted, and the third and fourth pluralities ofgrating elements are configured to switch into a diffracting state whenthe source of light emits the light of the second wavelength band.

In yet another additional embodiment again, the optical illuminationdevice further includes third and fourth pluralities of gratingelements, wherein the at least one waveguide includes first and secondgrating layers, the first and third pluralities of grating elements areinterspersed within the first grating layer, the second and fourthpluralities of grating elements are interspersed within the secondgrating layer, the light having a first polarization state includeslight of a first wavelength band and light of a second wavelength band,and the at least one input coupler includes a first input coupler forcoupling the light of the first wavelength band towards the first andsecond pluralities of grating elements, and a second input coupler forcoupling the light of the second wavelength band towards the third andfourth pluralities of grating elements.

In a yet further additional embodiment again, the optical illuminationdevice further includes a quarter-wave plate having a reflectivesurface, third and fourth pluralities of grating elements, wherein thesource of light is further configured to emit light having a secondpolarization state, the light having the first polarization state is ina first wavelength band, and the light having the second polarizationstate is in a second wavelength band, the third and fourth pluralitiesof grating elements are configured to diffract the light having thesecond polarization state towards the quarter-wave plate, the at leastone waveguide includes first and second grating layers, the first andthird pluralities of grating elements are interspersed within the firstgrating layer, and the second and fourth pluralities of grating elementsare interspersed within the second grating layer, and the firstplurality of grating elements spatially overlaps the second plurality ofgrating elements.

In still yet another additional embodiment again, the first plurality ofgrating elements has a grating vector in a first direction, and thesecond plurality of grating elements has a grating vector in an opposingdirection to the first direction.

In a still yet further additional embodiment again, the source of lightis a laser source.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention.

FIGS. 1A and 1B conceptually illustrate HPDLC SBG devices and theswitching property of SBGs in accordance with various embodiments of theinvention.

FIG. 2 conceptually illustrates a waveguide backlight in accordance withan embodiment of the invention.

FIG. 3 conceptually illustrates a flow chart of a process for providinga waveguide backlight in accordance with an embodiment of the invention.

FIG. 4 conceptually illustrates a waveguide backlight with two waveguidelayers in accordance with an embodiment of the invention.

FIG. 5 conceptually illustrates a waveguide backlight with a singlewaveguide layer in accordance with an embodiment of the invention.

FIG. 6 conceptually illustrates a waveguide backlight having twowaveguide layers with alternating wavelength-diffracting gratingelements in accordance with an embodiment of the invention.

FIG. 7 conceptually illustrates a waveguide backlight having a singlewaveguide layer with alternating wavelength-diffracting grating elementsin accordance with an embodiment of the invention.

FIG. 8 conceptually illustrates a waveguide backlight having twowaveguide layers with alternating wavelength-diffracting gratingelements for input light having orthogonal polarizations in accordancewith an embodiment of the invention.

FIG. 9 conceptually illustrates a waveguide backlight having a singlewaveguide layer with alternating wavelength-diffracting grating elementsfor input light having orthogonal polarizations in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

For the purposes of describing embodiments, some well-known features ofoptical technology known to those skilled in the art of optical designand visual displays have been omitted or simplified in order to notobscure the basic principles of the invention. Unless otherwise stated,the term “on-axis” in relation to a ray or a beam direction refers topropagation parallel to an axis normal to the surfaces of the opticalcomponents described in relation to the invention. In the followingdescription the terms light, ray, beam, and direction may be usedinterchangeably and in association with each other to indicate thedirection of propagation of electromagnetic radiation along rectilineartrajectories. The term light and illumination may be used in relation tothe visible and infrared bands of the electromagnetic spectrum. Parts ofthe following description will be presented using terminology commonlyemployed by those skilled in the art of optical design. As used herein,the term grating may encompass a grating comprised of a set of gratingsin some embodiments. For illustrative purposes, it is to be understoodthat the drawings are not drawn to scale unless stated otherwise.

An ideal backlight unit (BLU) should have a compact (i.e., thin) formfactor and should deliver uniform luminance and color with efficientcoupling of light from the illumination source and extraction from theBLU onto the display panel to be back-lit. In mobile displays, the BLUthickness should be a few millimeters. Television displays likewiserequire low thickness to image diagonal ratios. Traditional edge-litsolutions have failed to meet form factor and uniformity requirements.Waveguide or light guiding, which carry the illumination light by totalinternal reflection while extracting portions of such light from thewaveguide, can provide very thin form factors. However, waveguides cansuffer from spatial variations of luminance and color due to thedispersive properties of gratings typically implemented in waveguides.In some cases, dispersion can be greatly alleviated by using lasersources.

Illumination nonuniformities can arise from wavelength-dependentabsorption within gratings; the small loss incurred at each beam-gratinginteraction can be multiplied as the beam propagates down the waveguide,leading to a progressive dimming of light along the waveguide. Wherelaser sources are used, which can make for a very compact lightsource-to-waveguide coupling optics, the high coherence of the laserscan result in a banding effect caused by gaps or overlaps due toimperfect interlacing of the total internal reflection beams. Laser-litBLUs can also suffer from laser speckle. Another source ofnonuniformity, when birefringent materials are used to form gratings,results from polarization rotations occurring at each beam bounce. Thispolarization variation can manifest itself as luminance nonuniformity.Color nonuniformity can also occur due to wavelength dependence ofbirefringence. Finally, birefringent gratings can result in spatiallyvarying polarization at the output of the BLU. This can result inluminance and color nonuniformity when the display panel to be lit is aliquid crystal device.

Turning now to the drawings, holographic waveguide backlight inaccordance with various embodiments of the invention are illustrated. Inmany embodiments, the waveguide backlight is implemented as a compact,efficient, highly uniform, color waveguide backlight that can be used ina range of display applications, such as but not limited to LCDmonitors, digital holographic display, and mobile computing andtelecommunications devices. In many embodiments, the waveguide backlightincludes a waveguide and a source of light configured to provide inputlight. The input light can be coupled into the waveguide in a totalinternal reflection path using a variety of different methods. In someembodiments, an input coupler, such as but not limited to a grating or aprism, is utilized to couple light into the waveguide. In severalembodiments, the source of light is configured to provide light ofdifferent wavelengths. In further embodiments, the source of light isconfigured to emit at least first and second wavelength collimated lightcolor sequentially. The waveguide can include at least two sets ofgrating elements disposed across at least one grating layer. Each set ofgrating elements can be configured to operate at a specificwavelength/angular band. In many embodiments, each set of gratingelements is configured to diffract and extract either upward-going ordownward-going light. In several embodiments, each set of gratingelements are configured for a specific wavelength band. In furtherembodiments, each set of grating elements include switchable Bragggratings and is switched into a diffracting state when the light sourceemits wavelength light intended for that set. In some embodiments,waveplates and retarders are implemented to control the polarization oflight. As can readily be appreciated, waveguide backlights in accordancewith various embodiments of the invention can be implemented in numerousconfigurations, the specific of which can depend on the application.Waveguide backlight configurations, optical waveguide structures,materials, and manufacturing processes are discussed in the sectionsbelow in further detail.

Optical Waveguide and Grating Structures

Optical structures recorded in waveguides can include many differenttypes of optical elements, such as but not limited to diffractiongratings. Gratings can be implemented to perform various opticalfunctions, including but not limited to coupling light, directing light,and preventing the transmission of light. In many embodiments, thegratings are surface relief gratings that reside on the outer surface ofthe waveguide. In other embodiments, the grating implemented is a Bragggrating (also referred to as a volume grating), which are structureshaving a periodic refractive index modulation. Bragg gratings can befabricated using a variety of different methods. One process includesinterferential exposure of holographic photopolymer materials to formperiodic structures. Bragg gratings can have high efficiency with littlelight being diffracted into higher orders. The relative amount of lightin the diffracted and zero order can be varied by controlling therefractive index modulation of the grating, a property that can be usedto make lossy waveguide gratings for extracting light over a largepupil.

One class of Bragg gratings used in holographic waveguide devices is theSwitchable Bragg Grating (SBG). SBGs can be fabricated by first placinga thin film of a mixture of photopolymerizable monomers and liquidcrystal material between substrates. The substrates can be made ofvarious types of materials, such glass and plastics. In many cases, thesubstrates are in a parallel configuration. In other embodiments, thesubstrates form a wedge shape. One or both substrates can supportelectrodes, typically transparent tin oxide films, for applying anelectric field across the film. The grating structure in an SBG can berecorded in the liquid material (often referred to as the syrup) throughphotopolymerization-induced phase separation using interferentialexposure with a spatially periodic intensity modulation. Factors such asbut not limited to control of the irradiation intensity, componentvolume fractions of the materials in the mixture, and exposuretemperature can determine the resulting grating morphology andperformance. As can readily be appreciated, a wide variety of materialsand mixtures can be used depending on the specific requirements of agiven application. In many embodiments, HPDLC material is used. Duringthe recording process, the monomers polymerize, and the mixtureundergoes a phase separation. The LC molecules aggregate to formdiscrete or coalesced droplets that are periodically distributed inpolymer networks on the scale of optical wavelengths. The alternatingliquid crystal-rich and liquid crystal-depleted regions form the fringeplanes of the grating, which can produce Bragg diffraction with a strongoptical polarization resulting from the orientation ordering of the LCmolecules in the droplets.

The resulting volume phase grating can exhibit very high diffractionefficiency, which can be controlled by the magnitude of the electricfield applied across the film. When an electric field is applied to thegrating via transparent electrodes, the natural orientation of the LCdroplets can change, causing the refractive index modulation of thefringes to lower and the hologram diffraction efficiency to drop to verylow levels. Typically, the electrodes are configured such that theapplied electric field will be perpendicular to the substrates. In anumber of embodiments, the electrodes are fabricated from indium tinoxide (ITO). In the OFF state with no electric field applied, theextraordinary axis of the liquid crystals generally aligns normal to thefringes. The grating thus exhibits high refractive index modulation andhigh diffraction efficiency for P-polarized light. When an electricfield is applied to the HPDLC, the grating switches to the ON statewherein the extraordinary axes of the liquid crystal molecules alignparallel to the applied field and hence perpendicular to the substrate.In the ON state, the grating exhibits lower refractive index modulationand lower diffraction efficiency for both S- and P-polarized light.Thus, the grating region no longer diffracts light. Each grating regioncan be divided into a multiplicity of grating elements such as forexample a pixel matrix according to the function of the HPDLC device.Typically, the electrode on one substrate surface is uniform andcontinuous, while electrodes on the opposing substrate surface arepatterned in accordance to the multiplicity of selectively switchablegrating elements.

Typically, the SBG elements are switched clear in 30 μs with a longerrelaxation time to switch ON. The diffraction efficiency of the devicecan be adjusted, by means of the applied voltage, over a continuousrange. In many cases, the device exhibits near 100% efficiency with novoltage applied and essentially zero efficiency with a sufficiently highvoltage applied. In certain types of HPDLC devices, magnetic fields canbe used to control the LC orientation. In some HPDLC applications, phaseseparation of the LC material from the polymer can be accomplished tosuch a degree that no discernible droplet structure results. An SBG canalso be used as a passive grating. In this mode, its chief benefit is auniquely high refractive index modulation. SBGs can be used to providetransmission or reflection gratings for free space applications. SBGscan be implemented as waveguide devices in which the HPDLC forms eitherthe waveguide core or an evanescently coupled layer in proximity to thewaveguide. The substrates used to form the HPDLC cell provide a totalinternal reflection (TIR) light guiding structure. Light can be coupledout of the SBG when the switchable grating diffracts the light at anangle beyond the TIR condition. In a number of embodiments, a reversemode grating device can be implemented—i.e., the grating is in itsnon-diffracting (cleared) state when the applied voltage is zero andswitches to its diffracting stated when a voltage is applied across theelectrodes.

FIGS. 1A and 1B conceptually illustrate HPDLC SBG devices 100, 110 andthe switching property of SBGs in accordance with various embodiments ofthe invention. In FIG. 1A, the SBG 100 is in an OFF state. As shown, theLC molecules 101 are aligned substantially normal to the fringe planes.As such, the SBG 100 exhibits high diffraction efficiency, and incidentlight can easily be diffracted. FIG. 1B illustrates the SBG 110 in an ONposition. An applied voltage 111 can orient the optical axis of the LCmolecules 112 within the droplets 113 to produce an effective refractiveindex that matches the polymer's refractive index, essentially creatinga transparent cell where incident light is not diffracted. In theillustrative embodiment, an AC voltage source is shown. As can readilybe appreciated, various voltage sources can be utilized depending on thespecific requirements of a given application. Furthermore, differentmaterials and device configurations can also be implemented. In someembodiments, the device implements different material systems and canoperate in reverse with respect to the applied voltage—i.e., the deviceexhibits high diffraction efficiency in response to an applied voltage.

In some embodiments, LC can be extracted or evacuated from the SBG toprovide a surface relief grating (SRG) that has properties very similarto a Bragg grating due to the depth of the SRG structure (which is muchgreater than that practically achievable using surface etching and otherconventional processes commonly used to fabricate SRGs). The LC can beextracted using a variety of different methods, including but notlimited to flushing with isopropyl alcohol and solvents. In manyembodiments, one of the transparent substrates of the SBG is removed,and the LC is extracted. In further embodiments, the removed substrateis replaced. The SRG can be at least partially backfilled with amaterial of higher or lower refractive index. Such gratings offer scopefor tailoring the efficiency, angular/spectral response, polarization,and other properties to suit various waveguide applications.

Waveguides in accordance with various embodiments of the invention caninclude various grating configurations designed for specific purposesand functions. In many embodiments, the waveguide is designed toimplement a grating configuration capable of preserving eyebox sizewhile reducing lens size by effectively expanding the exit pupil of acollimating optical system. The exit pupil can be defined as a virtualaperture where only the light rays which pass though this virtualaperture can enter the eyes of a user. In some embodiments, thewaveguide includes an input grating optically coupled to a light source,a fold grating for providing a first direction beam expansion, and anoutput grating for providing beam expansion in a second direction, whichis typically orthogonal to the first direction, and beam extractiontowards the eyebox. As can readily be appreciated, the gratingconfiguration implemented waveguide architectures can depend on thespecific requirements of a given application. In some embodiments, thegrating configuration includes multiple fold gratings. In severalembodiments, the grating configuration includes an input grating and asecond grating for performing beam expansion and beam extractionsimultaneously. The second grating can include gratings of differentprescriptions, for propagating different portions of the field-of-view,arranged in separate overlapping grating layers or multiplexed in asingle grating layer. Multiplexed gratings can include thesuperimposition of at least two gratings having different gratingprescriptions within the same volume. Gratings having different gratingprescriptions can have different grating vectors and/or grating slantwith respect to the waveguide's surface.

In several embodiments, the gratings within each layer are designed tohave different spectral and/or angular responses. For example, in manyembodiments, different gratings across different grating layers areoverlapped, or multiplexed, to provide an increase in spectralbandwidth. In some embodiments, a full color waveguide is implementedusing three grating layers, each designed to operate in a differentspectral band (red, green, and blue). In other embodiments, a full colorwaveguide is implemented using two grating layers, a red-green gratinglayer and a green-blue grating layer. As can readily be appreciated,such techniques can be implemented similarly for increasing angularbandwidth operation of the waveguide. In addition to the multiplexing ofgratings across different grating layers, multiple gratings can bemultiplexed within a single grating layer—i.e., multiple gratings can besuperimposed within the same volume. In several embodiments, thewaveguide includes at least one grating layer having two or more gratingprescriptions multiplexed in the same volume. In further embodiments,the waveguide includes two grating layers, each layer having two gratingprescriptions multiplexed in the same volume. Multiplexing two or moregrating prescriptions within the same volume can be achieved usingvarious fabrication techniques. In a number of embodiments, amultiplexed master grating is utilized with an exposure configuration toform a multiplexed grating. In many embodiments, a multiplexed gratingis fabricated by sequentially exposing an optical recording materiallayer with two or more configurations of exposure light, where eachconfiguration is designed to form a grating prescription. In someembodiments, a multiplexed grating is fabricated by exposing an opticalrecording material layer by alternating between or among two or moreconfigurations of exposure light, where each configuration is designedto form a grating prescription. As can readily be appreciated, varioustechniques, including those well known in the art, can be used asappropriate to fabricate multiplexed gratings.

In many embodiments, the waveguide can incorporate at least one of:angle multiplexed gratings, color multiplexed gratings, fold gratings,dual interaction gratings, rolled K-vector gratings, crossed foldgratings, tessellated gratings, chirped gratings, gratings withspatially varying refractive index modulation, gratings having spatiallyvarying grating thickness, gratings having spatially varying averagerefractive index, gratings with spatially varying refractive indexmodulation tensors, and gratings having spatially varying averagerefractive index tensors. In some embodiments, the waveguide canincorporate at least one of: a half wave plate, a quarter wave plate, ananti-reflection coating, a beam splitting layer, an alignment layer, aphotochromic back layer for glare reduction, and louvre films for glarereduction. In several embodiments, the waveguide can support gratingsproviding separate optical paths for different polarizations. In variousembodiments, the waveguide can support gratings providing separateoptical paths for different spectral bandwidths. In a number ofembodiments, the gratings can be HPDLC gratings, switching gratingsrecorded in HPDLC (such switchable Bragg Gratings), Bragg gratingsrecorded in holographic photopolymer, or surface relief gratings. Inmany embodiments, the waveguide operates in a monochrome band. In someembodiments, the waveguide operates in the green band. In severalembodiments, waveguide layers operating in different spectral bands suchas red, green, and blue (RGB) can be stacked to provide a three-layerwaveguiding structure. In further embodiments, the layers are stackedwith air gaps between the waveguide layers. In various embodiments, thewaveguide layers operate in broader bands such as blue-green andgreen-red to provide two-waveguide layer solutions. In otherembodiments, the gratings are color multiplexed to reduce the number ofgrating layers. Various types of gratings can be implemented. In someembodiments, at least one grating in each layer is a switchable grating.

Waveguides incorporating optical structures such as those discussedabove can be implemented in a variety of different applications,including but not limited to waveguide displays. In various embodiments,the waveguide display is implemented with an eyebox of greater than 10mm with an eye relief greater than 25 mm. In some embodiments, thewaveguide display includes a waveguide with a thickness between 2.0-5.0mm. In many embodiments, the waveguide display can provide an imagefield-of-view of at least 50° diagonal. In further embodiments, thewaveguide display can provide an image field-of-view of at least 70°diagonal. The waveguide display can employ many different types ofpicture generation units (PGUs). In several embodiments, the PGU can bea reflective or transmissive spatial light modulator such as a liquidcrystal on Silicon (LCoS) panel or a micro electromechanical system(MEMS) panel. In a number of embodiments, the PGU can be an emissivedevice such as an organic light emitting diode (OLED) panel. In someembodiments, an OLED display can have a luminance greater than 4000 nitsand a resolution of 4 k×4 k pixels. In several embodiments, thewaveguide can have an optical efficiency greater than 10% such that agreater than 400 nit image luminance can be provided using an OLEDdisplay of luminance 4000 nits. Waveguides implementing P-diffractinggratings (i.e., gratings with high efficiency for P-polarized light)typically have a waveguide efficiency of 5%-6.2%. Since P-diffracting orS-diffracting gratings can waste half of the light from an unpolarizedsource such as an OLED panel, many embodiments are directed towardswaveguides capable of providing both S-diffracting and P-diffractinggratings to allow for an increase in the efficiency of the waveguide byup to a factor of two. In some embodiments, the S-diffracting andP-diffracting gratings are implemented in separate overlapping gratinglayers. Alternatively, a single grating can, under certain conditions,provide high efficiency for both p-polarized and s-polarized light. Inseveral embodiments, the waveguide includes Bragg-like gratings producedby extracting LC from HPDLC gratings, such as those described above, toenable high S and P diffraction efficiency over certain wavelength andangle ranges for suitably chosen values of grating thickness (typically,in the range 2-5 μm).

Optical Recording Material Systems

HPDLC mixtures generally include LC, monomers, photoinitiator dyes, andcoinitiators. The mixture (often referred to as syrup) frequently alsoincludes a surfactant. For the purposes of describing the invention, asurfactant is defined as any chemical agent that lowers the surfacetension of the total liquid mixture. The use of surfactants in PDLCmixtures is known and dates back to the earliest investigations ofPDLCs. For example, a paper by R. L Sutherland et al., SPIE Vol. 2689,158-169, 1996, the disclosure of which is incorporated herein byreference, describes a PDLC mixture including a monomer, photoinitiator,coinitiator, chain extender, and LCs to which a surfactant can be added.Surfactants are also mentioned in a paper by Natarajan et al, Journal ofNonlinear Optical Physics and Materials, Vol. 5 No. I 89-98, 1996, thedisclosure of which is incorporated herein by reference. Furthermore,U.S. Pat. No. 7,018,563 by Sutherland; et al., discussespolymer-dispersed liquid crystal material for forming apolymer-dispersed liquid crystal optical element having: at least oneacrylic acid monomer; at least one type of liquid crystal material; aphotoinitiator dye; a coinitiator; and a surfactant. The disclosure ofU.S. Pat. No. 7,018,563 is hereby incorporated by reference in itsentirety.

The patent and scientific literature contains many examples of materialsystems and processes that can be used to fabricate SBGs, includinginvestigations into formulating such material systems for achieving highdiffraction efficiency, fast response time, low drive voltage, and soforth. U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No.5,751,452 by Tanaka et al. both describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices. Examples ofrecipes can also be found in papers dating back to the early 1990s. Manyof these materials use acrylate monomers, including:

-   R. L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the disclosure    of which is incorporated herein by reference, describes the use of    acrylate polymers and surfactants. Specifically, the recipe includes    a crosslinking multifunctional acrylate monomer; a chain extender    N-vinyl pyrrolidinone, LC E7, photo-initiator rose Bengal, and    coinitiator N-phenyl glycine. Surfactant octanoic acid was added in    certain variants.-   Fontecchio et al., SID 00 Digest 774-776, 2000, the disclosure of    which is incorporated herein by reference, describes a UV curable    HPDLC for reflective display applications including a    multi-functional acrylate monomer, LC, a photoinitiator, a    coinitiators, and a chain terminator.-   Y. H. Cho, et al., Polymer International, 48, 1085-1090, 1999, the    disclosure of which is incorporated herein by reference, discloses    HPDLC recipes including acrylates.-   Karasawa et al., Japanese Journal of Applied Physics, Vol. 36,    6388-6392, 1997, the disclosure of which is incorporated herein by    reference, describes acrylates of various functional orders.-   T. J. Bunning et al., Polymer Science: Part B: Polymer Physics, Vol.    35, 2825-2833, 1997, the disclosure of which is incorporated herein    by reference, also describes multifunctional acrylate monomers.-   G. S. Iannacchione et al., Europhysics Letters Vol. 36 (6). 425-430,    1996, the disclosure of which is incorporated herein by reference,    describes a PDLC mixture including a penta-acrylate monomer, LC,    chain extender, coinitiators, and photoinitiator.    Acrylates offer the benefits of fast kinetics, good mixing with    other materials, and compatibility with film forming processes.    Since acrylates are cross-linked, they tend to be mechanically    robust and flexible. For example, urethane acrylates of    functionality 2 (di) and 3 (tri) have been used extensively for    HPDLC technology. Higher functionality materials such as penta and    hex functional stems can also be used.

Modulation of Material Composition

High luminance and excellent color fidelity are important factors invarious waveguide applications. In each case, high uniformity across theFOV can be desired. However, the fundamental optics of waveguides canlead to non-uniformities due to gaps or overlaps of beams bouncing downthe waveguide. Further non-uniformities may arise from imperfections inthe gratings and non-planarity of the waveguide substrates. In SBGs,there can exist a further issue of polarization rotation by birefringentgratings. In applicable cases, the biggest challenge is usually the foldgrating where there are millions of light paths resulting from multipleintersections of the beam with the grating fringes. Careful managementof grating properties, particularly the refractive index modulation, canbe utilized to overcome non-uniformity.

Out of the multitude of possible beam interactions (diffraction or zeroorder transmission), only a subset contributes to the signal presentedat the eye box. By reverse tracing from the eyebox, fold regionscontributing to a given field point can be pinpointed. The precisecorrection to the modulation that is needed to send more into the darkregions of the output illumination can then be calculated. Havingbrought the output illumination uniformity for one color back on target,the procedure can be repeated for other colors. Once the indexmodulation pattern has been established, the design can be exported tothe deposition mechanism, with each target index modulation translatingto a unique deposition setting for each spatial resolution cell on thesubstrate to be coated/deposited. The resolution of the depositionmechanism can depend on the technical limitations of the systemutilized. In many embodiments, the spatial pattern can be implemented to30 micrometers resolution with full repeatability.

Compared with waveguides utilizing surface relief gratings (SRGs), SBGwaveguides implementing manufacturing techniques in accordance withvarious embodiments of the invention can allow for the grating designparameters that impact efficiency and uniformity, such as but notlimited to refractive index modulation and grating thickness, to beadjusted dynamically during the deposition process without the need fora different master. With SRGs where modulation is controlled by etchdepth, such schemes would not be practical as each variation of thegrating would entail repeating the complex and expensive toolingprocess. Additionally, achieving the required etch depth precision andresist imaging complexity can be very difficult.

Deposition processes in accordance with various embodiments of theinvention can provide for the adjustment of grating design parameters bycontrolling the type of material that is to be deposited. Variousembodiments of the invention can be configured to deposit differentmaterials, or different material compositions, in different areas on thesubstrate. For example, deposition processes can be configured todeposit HPDLC material onto an area of a substrate that is meant to be agrating region and to deposit monomer onto an area of the substrate thatis meant to be a non-grating region. In several embodiments, thedeposition process is configured to deposit a layer of optical recordingmaterial that varies spatially in component composition, allowing forthe modulation of various aspects of the deposited material. Thedeposition of material with different compositions can be implemented inseveral different ways. In many embodiments, more than one depositionhead can be utilized to deposit different materials and mixtures. Eachdeposition head can be coupled to a different material/mixturereservoir. Such implementations can be used for a variety ofapplications. For example, different materials can be deposited forgrating and non-grating areas of a waveguide cell. In some embodiments,HPDLC material is deposited onto the grating regions while only monomeris deposited onto the non-grating regions. In several embodiments, thedeposition mechanism can be configured to deposit mixtures withdifferent component compositions.

In some embodiments, spraying nozzles can be implemented to depositmultiple types of materials onto a single substrate. In waveguideapplications, the spraying nozzles can be used to deposit differentmaterials for grating and non-grating areas of the waveguide. In manyembodiments, the spraying mechanism is configured for printing gratingsin which at least one the material composition, birefringence, and/orthickness can be controlled using a deposition apparatus having at leasttwo selectable spray heads. In some embodiments, the manufacturingsystem provides an apparatus for depositing grating recording materialoptimized for the control of laser banding. In several embodiments, themanufacturing system provides an apparatus for depositing gratingrecording material optimized for the control of polarizationnon-uniformity. In several embodiments, the manufacturing systemprovides an apparatus for depositing grating recording materialoptimized for the control of polarization non-uniformity in associationwith an alignment control layer. In a number of embodiments, thedeposition workcell can be configured for the deposition of additionallayers such as beam splitting coatings and environmental protectionlayers. Inkjet print heads can also be implemented to print differentmaterials in different regions of the substrate.

As discussed above, deposition processes can be configured to depositoptical recording material that varies spatially in componentcomposition. Modulation of material composition can be implemented inmany different ways. In a number of embodiments, an inkjet print headcan be configured to modulate material composition by utilizing thevarious inkjet nozzles within the print head. By altering thecomposition on a “dot-by-dot” basis, the layer of optical recordingmaterial can be deposited such that it has a varying composition acrossthe planar surface of the layer. Such a system can be implemented usinga variety of apparatuses including but not limited to inkjet printheads. Similar to how color systems use a palette of only a few colorsto produce a spectrum of millions of discrete color values, such as theCMYK system in printers or the additive RGB system in displayapplications, inkjet print heads in accordance with various embodimentsof the invention can be configured to print optical recording materialswith varying compositions using only a few reservoirs of differentmaterials. Different types of inkjet print heads can have differentprecision levels and can print with different resolutions. In manyembodiments, a 300 DPI (“dots per inch”) inkjet print head is utilized.Depending on the precision level, discretization of varying compositionsof a given number of materials can be determined across a given area.For example, given two types of materials to be printed and an inkjetprint head with a precision level of 300 DPI, there are 90,001 possiblediscrete values of composition ratios of the two types of materialsacross a square inch for a given volume of printed material if each dotlocation can contain either one of the two types of materials. In someembodiments, each dot location can contain either one of the two typesof materials or both materials. In several embodiments, more than oneinkjet print head is configured to print a layer of optical recordingmaterial with a spatially varying composition. Although the printing ofdots in a two-material application is essentially a binary system,averaging the printed dots across an area can allow for discretizationof a sliding scale of ratios of the two materials to be printed. Forexample, the amount of discrete levels of possible concentrations/ratiosacross a unit square is given by how many dot locations can be printedwithin the unit square. As such, there can be a range of differentconcentration combinations, ranging from 100% of the first material to100% of the second material. As can readily be appreciated, the conceptsare applicable to real units and can be determined by the precisionlevel of the inkjet print head. Although specific examples of modulatingthe material composition of the printed layer are discussed, the conceptof modulating material composition using inkjet print heads can beexpanded to use more than two different material reservoirs and can varyin precision levels, which largely depends on the types of print headsused.

Varying the composition of the material printed can be advantageous forseveral reasons. For example, in many embodiments, varying thecomposition of the material during deposition can allow for theformation of a waveguide with gratings that have spatially varyingdiffraction efficiencies across different areas of the gratings. Inembodiments utilizing HPDLC mixtures, this can be achieved by modulatingthe relative concentration of liquid crystals in the HPDLC mixtureduring the printing process, which creates compositions that can producegratings with varying diffraction efficiencies when the material isexposed. In several embodiments, a first HPDLC mixture with a certainconcentration of liquid crystals and a second HPDLC mixture that isliquid crystal-free are used as the printing palette in an inkjet printhead for modulating the diffraction efficiencies of gratings that can beformed in the printed material. In such embodiments, discretization canbe determined based on the precision of the inkjet print head. Adiscrete level can be given by the concentration/ratio of the materialsprinted across a certain area. In this example, the discrete levelsrange from no liquid crystal to the maximum concentration of liquidcrystals in the first PDLC mixture.

The ability to vary the diffraction efficiency across a waveguide can beused for various purposes. A waveguide is typically designed to guidelight internally by reflecting the light many times between the twoplanar surfaces of the waveguide. These multiple reflections can allowfor the light path to interact with a grating multiple times. In manyembodiments, a layer of material can be printed with varying compositionof materials such that the gratings formed have spatially varyingdiffraction efficiencies to compensate for the loss of light duringinteractions with the gratings to allow for a uniform output intensity.For example, in some waveguide applications, an output grating isconfigured to provide exit pupil expansion in one direction while alsocoupling light out of the waveguide. The output grating can be designedsuch that when light within the waveguide interact with the grating,only a percentage of the light is refracted out of the waveguide. Theremaining portion continues in the same light path, which remains withinTIR and continues to be reflected within the waveguide. Upon a secondinteraction with the same output grating again, another portion of lightis refracted out of the waveguide. During each refraction, the amount oflight still traveling within the waveguide decreases by the amountrefracted out of the waveguide. As such, the portions refracted at eachinteraction gradually decreases in terms of total intensity. By varyingthe diffraction efficiency of the grating such that it increases withpropagation distance, the decrease in output intensity along eachinteraction can be compensated, allowing for a uniform output intensity.

Varying the diffraction efficiency can also be used to compensate forother attenuation of light within a waveguide. All objects have a degreeof reflection and absorption. Light trapped in TIR within a waveguideare continually reflected between the two surfaces of the waveguide.Depending on the material that makes up the surfaces, portions of lightcan be absorbed by the material during each interaction. In many cases,this attenuation is small, but can be substantial across a large areawhere many reflections occur. In many embodiments, a waveguide cell canbe printed with varying compositions such that the gratings formed fromthe optical recording material layer have varying diffractionefficiencies to compensate for the absorption of light from thesubstrates. Depending on the substrates, certain wavelengths can be moreprone to absorption by the substrates. In a multi-layered waveguidedesign, each layer can be designed to couple in a certain range ofwavelengths of light. Accordingly, the light coupled by these individuallayers can be absorbed in different amounts by the substrates of thelayers. For example, in a number of embodiments, the waveguide is madeof a three-layered stack to implement a full color display, where eachlayer is designed for one of red, green, and blue. In such embodiments,gratings within each of the waveguide layers can be formed to havevarying diffraction efficiencies to perform color balance optimizationby compensating for color imbalance due to loss of transmission ofcertain wavelengths of light.

In addition to varying the liquid crystal concentration within thematerial in order to vary the diffraction efficiency, another techniqueincludes varying the thickness of the waveguide cell. This can beaccomplished through the use of spacers. In many embodiments, spacersare dispersed throughout the optical recording material for structuralsupport during the construction of the waveguide cell. In someembodiments, different sizes of spacers are dispersed throughout theoptical recording material. The spacers can be dispersed in ascendingorder of sizes across one direction of the layer of optical recordingmaterial. When the waveguide cell is constructed through lamination, thesubstrates sandwich the optical recording material and, with structuralsupport from the varying sizes of spacers, create a wedge-shaped layerof optical recording material. spacers of varying sizes can be dispersedsimilar to the modulation process described above. Additionally,modulating spacer sizes can be combined with modulation of materialcompositions. In several embodiments, reservoirs of HPDLC materials eachsuspended with spacers of different sizes are used to print a layer ofHPDLC material with spacers of varying sizes strategically dispersed toform a wedge-shaped waveguide cell. In a number of embodiments, spacersize modulation is combined with material composition modulation byproviding a number of reservoirs equal to the product of the number ofdifferent sizes of spacers and the number of different materials used.For example, in one embodiment, the inkjet print head is configured toprint varying concentrations of liquid crystal with two different spacersizes. In such an embodiment, four reservoirs can be prepared: a liquidcrystal-free mixture suspension with spacers of a first size, a liquidcrystal-free mixture-suspension with spacers of a second size, a liquidcrystal-rich mixture-suspension with spacers of a first size, and aliquid crystal-rich mixture-suspension with spacers of a second size.Further discussion regarding material modulation can be found in U.S.application Ser. No. 16/203,071 filed Nov. 18, 2018 entitled “Systemsand Methods for Manufacturing Waveguide Cells.” The disclosure of U.S.application Ser. No. 16/203,491 is hereby incorporated by reference inits entirety for all purposes.

Waveguide Backlights

Waveguide backlights in accordance with various embodiments of theinvention can be implemented using a variety of differentconfigurations. As can readily be appreciated, the specificconfiguration implemented can depend on various factors, including butnot limited to the intended application, cost constraints, form factorconstraints, etc. In many embodiments, the waveguide backlight isimplemented with at least one waveguide layer containing at least onegrating layer sandwiched by first and second substrates. The substratescan include various transparent materials, including but not limited toglass and plastics. The grating layer(s) can include different sets ofgrating elements configured for various purposes. In some embodiments,the grating layer includes two different sets of grating elements, eachset configured and designed to have high diffraction efficiency for aspecific wavelength band and/or angular band. In a number ofembodiments, the grating layer includes two different sets of gratingelements, where each set contains grating elements having the sameK-vectors. In various embodiments, the two sets of grating elements haveopposing K-vectors. In several embodiments, the grating layer includestwo different sets of grating elements, each set configured and designedto diffract and extract light from different directions. For example, ina number of embodiments, the grating layer includes a first set ofgrating elements configured to diffract TIR light that is reflected offthe first substrate and to extract such light through the secondsubstrate and a second set of grating elements configured to diffractTIR light that is reflected off the second substrate and to extract suchlight through the first substrate.

The grating elements implemented in waveguide backlights can be arrangedin a number of different configurations. In many embodiments, thewaveguide backlight includes a grating layer having first and secondsets of grating elements that are interspersed with one another. In someembodiments, the first and second set of grating elements are disposedacross two different grating layers. The two different grating layerscan be disposed adjacent one another (i.e., the waveguide layer includestwo grating layers sandwiched between two substrates) or across twodifferent waveguide layers. As can readily be appreciated, such gratingarchitectures can be expanded to implement more than two sets of gratingelements. Furthermore, the waveguide layer(s) can be configured toimplement a variety of different grating structures, including but notlimited to HPDLC gratings, switching gratings recorded in HPDLC (suchswitchable Bragg Gratings), Bragg gratings recorded in holographicphotopolymer, evacuated Bragg gratings, backfilled evacuated Bragggratings, and surface relief gratings.

Depending on the type of gratings implemented, light polarizationresponses can be a large factor in how and how well the waveguidebacklight operates. For example, in some embodiments, the gratings areimplemented using an HPDLC material that forms gratings that aresensitive to P-polarized light. In such cases, the waveguide backlightcan be designed with the appropriate considerations. The waveguidebacklight can include various waveplate and retarder configurations formanipulating the polarization of light traveling throughout thewaveguide backlight. In some embodiments, the waveguide backlightincludes a quarter-wave plate (QWP). A QWP converts linearly polarizedlight into circularly polarized light and vice versa. In furtherembodiments, the QWP is implemented with a mirror, which can be formedon an outer surface of the QWP. Such configurations can allow forincident linearly polarized light to be reflected with its polarizationorthogonally changed. For example, an incident P-polarized light ray canbe converted into circularly polarized light by the QWP, reflected bythe mirror to give circularly polarized light in an opposing direction,and finally converted into linearly S-polarized light. In manyembodiments, the waveguide backlight includes a half-wave plate (HWP)for switching the polarization of P-polarized light into S-polarizedlight and vice versa. In a number of embodiments, the waveguidebacklight includes a substrate supporting half wave retarders. Varioustypes of light sources can be utilized to introduce light into thebacklight. In a number of embodiments, P- and/or S-polarized light iscoupled into the waveguide backlight. In several embodiments,unpolarized light is coupled into the waveguide backlight. As canreadily be appreciated, the specific configuration of input light andgrating structures can depend on the specific requirements of a givenapplication.

Grating elements within a waveguide backlight can be arranged andimplemented in various configurations. In several embodiments, all ofthe grating elements in a waveguide layer are designed to operate at acommon wavelength band. As described above, the grating elements canhave K-vectors configured to diffract upward-going or downward-goingrays in a waveguide layer. In several embodiments, both types ofgratings are provided in a waveguide layer. In further embodiments, bothtypes of gratings are provided in a single grating layer. In someembodiments, the grating elements can have K-vectors in differingdirections but operating at a common wavelength band. It should beappreciated from the discussions that any number of separate wavelengthbands can be provided. FIG. 2 conceptually illustrates a waveguidebacklight having two sets of interspersed grating elements in accordancewith an embodiment of the invention. As shown, the waveguide backlight200 includes: a waveguide 201 formed by substrates 202,203 sandwiching agrating layer 204. In many embodiments, a source of light (which is notillustrated) can be optically coupled to the waveguide structure 201 andcan be configured to emit collimated light. The substrates 202,203 canprovide a TIR structure for the input light. The grating layer 204 caninclude a plurality of grating elements for diffracting light out of thewaveguide and, ultimately, towards an external illumination surface. Inthe illustrative embodiment, the grating layer 204 includes two sets ofplane gratings having two grating configurations (e.g., grating elements205,206) with opposing K-vectors for diffracting TIR light coming fromdifferent directions. For example, grating element 205 is configured todiffract light reflected from the outer surface of substrate 202 whilegrating element 206 is configured to diffract light reflected from theouter surface of substrate 203. For ease of clarity, the two differentdirections of light can also be referred to as upward- anddownward-going TIR light, respectively, with the orientation of thewaveguide in the figure as a frame of reference. The pair of gratingconfigurations are repeated along the grating layer 204 in theembodiment of FIG. 2 to form two sets of interspersed grating elements.

The waveguide backlight 200 of FIG. 2 further includes a quarter waveplate 207 and a transparent layer 208 divided into clear regions 209 andregions supporting half wave retarders 210. In the illustrativeembodiment, the QWP 207 is implemented along with a mirror to providereflection of incident light while changing its polarizationorthogonally. The QWP 207 and the transparent layer 208 can be separatedfrom the waveguide 201 by air gaps or layers of low refractive indexmaterial, including but not limited to a nanoporous material. Methods ofsuch implementations are discussed in the sections above. Referring backto FIG. 2 , the illustrative embodiment shows operation of the waveguidebacklight 200 where input P-polarized light 211 (which is the preferredlight polarization state for diffraction by SBGs) undergoes TIR withinthe waveguide 201. A portion of this light (upward-going TIR light) canbe diffracted (212) by grating element 205 and directed towards a clearregion 209 of the transparent layer 208 to provide P-polarized outputlight 213. Downward-going TIR light incident on the grating element 206can be diffracted (214) downwards and reflected (215) by the QWP 207with its polarization rotated from P to S. The S-polarized light cantravel through the grating layer 204 and proceed out of the waveguide201 towards a half wave retarder region 210 of the transparent layer208. After transmission through a half wave retarder region 210, thelight has its polarization rotated from S to P (216). By repeating theabove ray-grating interactions across the waveguide, extraction of theincident light towards a similar direction can be accomplished to a highdegree. As can readily be appreciated, such configurations can includevarious modifications, which can depend on the specific requirements ofa given application. For example, in several embodiments, thediffraction efficiencies of the grating elements can be varied along thewaveguide path to control uniformity. In many embodiments, the gratingelements can be electrically switchable. In some embodiments, a gratinglayer can be formed between transparent substrates with transparentconductive coatings applied to each substrate. At least one of thecoatings can be patterned into independently addressable elementsoverlapping the grating elements. An electrical control circuitoperative to apply voltages across each of the grating elements can beprovided.

FIG. 3 conceptually illustrates a flow chart of a process for providinga waveguide backlight in accordance with an embodiment of the invention.As shown, the process 300 includes providing (301) a waveguide having afirst set of grating elements for diffracting downward-going rays and asecond set of grating elements diffracting upward-going rays, whereinthe grating elements are disposed between first and second transparentsubstrates. Input light can be coupled (302) into a total internalreflection path within the waveguide. Various types of input light canbe utilized. In many embodiments, narrow band laser illumination isutilized. In some embodiments, the input light is P-polarized light. Aportion of the input light can be extracted (303) through an outersurface of the first transparent substrate using the first set ofgrating elements, and a portion of the input light can be extracted(304) through an outer surface of the second transparent substrate usingthe second set of grating elements. In many embodiments, the first setof grating elements is configured to extract light reflected from theouter surface of the second substrate, and the second set of gratingelements is configured to extract light reflected from the outer surfaceof the first substrate. Various types of gratings can be implemented. Inseveral embodiments, P-polarization sensitive gratings are utilized. Ina number of embodiments, S-polarization sensitive gratings are utilized.In some embodiments, both types of gratings are implemented. As canreadily be appreciated, the types of gratings utilized can depend on thetype of input light. The light extracted from the second transparentsurface can have its polarization rotated (305) and can be reflectedtowards the waveguide, propagating through to the outer surface of thefirst transparent surface. In many embodiments, a QWP is utilized torotate the polarization of the light and to reflect it towards thewaveguide. The light with its polarization rotated can optionally haveits polarization rotated (306) again after its propagation through theouter surface of the first transparent substrate. In some embodiments, asubstrate containing HWP regions can be implemented to rotate thepolarization of the light after its propagation through the firsttransparent substrate. Although FIG. 3 illustrates a specific method ofproviding a waveguide backlight, various other processes can beimplemented as appropriate depending on the specific requirements of agiven application. For example, in some embodiments, the input lightcontains only P-polarized light. In other embodiments, the input lightcontains both S- and P-polarized light.

Waveguide backlights in accordance with various embodiments of theinvention can be configured for many different applications. In manyembodiments, the waveguide backlight is configured for narrow bandillumination applications—i.e., the wavelength band can have a narrowbandwidth as is typically provided by a laser. In some embodiments, thewavelength band can have a broader bandwidth such as can be provided byan LED. As can readily be appreciated, the backlight can also be used toprovide non-visible radiation such as infrared and ultraviolet. In someembodiments, the waveguide backlight is configured as a color waveguidebacklight. Such backlights can be implemented based on principlessimilar to those shown in FIG. 2 . In some embodiments, the backlightprovides light from red, green, and blue (RGB) sources. In suchembodiments, the backlight can include RGB grating elements interspersedwithin a single layer or disposed in some way over two or more layers.In some embodiments, separate RGB layers can be used. In severalembodiments, the waveguide backlight operates using first and secondwavelength input light that covers a large portion of the visible band.For example, in various embodiments, the first wavelength light coversthe blue to green band, and the second wavelength light can cover thegreen to red band. In several embodiments, a color waveguide backlightcan be implemented utilizing separate grating layers for each colorcomponent to be emitted from the backlight. In some embodiments, thewaveguide backlight incorporates SBGs. In such cases, the waveguidebacklight can include a first set of grating elements configured toswitch into a diffracting state when the light source emits light of afirst wavelength band and a second set of grating elements configured toswitch into a diffracting state when the source emits light of a secondwavelength band.

FIG. 4 conceptually illustrates a waveguide backlight with two waveguidelayers in accordance with an embodiment of the invention. In thefollowing paragraphs, to simplify the description of the invention, thediscussions will include waveguides for emitting light in two differentwavelength bands (first and second wavelength light) using first andsecond sets of grating elements formed in two waveguide layers, eachwaveguide layer containing a single grating layer. However, any numberof waveguides layers and grating layers can be utilized as appropriateddepending on the specific requirements of a given application. Referringback to FIG. 4 , the waveguide backlight 400 shown includes first andsecond waveguides 401,402. The backlight 400 further includes a quarterwave plate (QWP) 403 and a transparent substrate 404 divided into clearregions 405 and regions supporting half wave retarders 406. Eachwaveguide can be configured to operate according to principles similarto those shown in FIG. 2 . For example, the first waveguide 401 can beconfigured to receive p-polarized light of a first wavelength band, andthe second waveguide 402 can be configured to receive p-polarized lightof a second wavelength band. In the illustrative embodiment, each of thefirst and second waveguides 401,402 includes a grating layer 407,408.The second waveguide 402 can include a similar configuration of gratingelements to that of the first waveguide 401 but operating in a differentwavelength band. For example, in the illustrative embodiment, the firstwaveguide 401 can include grating elements configured to operate in thered-green wavelength band while the second waveguide 402 can includegrating elements configured to operate in the green-blue wavelengthband, allowing for the implementation of a full color waveguidebacklight. In other embodiments, a full color waveguide backlight can beimplemented with three waveguide layers, each configured to operate inone of red, green, and blue wavelength band. As indicated by the twosets of rays (dashed and solid, representing rays of differentwavelength light), it is shown that the ray and grating interactions ofthe second waveguide 402 are similar to those of the first waveguide401. The first and second wavelength light extracted from the twowaveguides can be combined to provide uniform illumination. In manyembodiments, the first and second wavelength light can be introduced tothe waveguides sequentially.

The waveguide structure of FIG. 4 can be equivalently implemented usinga variety of different grating configurations. In some embodiments, thebacklight can be implemented as a single waveguide layer containingpluralities of grating elements configured for operation at differentwavelength bands. FIG. 5 conceptually illustrates a waveguide backlightwith a single waveguide layer in accordance with an embodiment of theinvention. As shown, the waveguide backlight 500 includes a gratingconfiguration 501 formed of two adjacent grating layers 502,503sandwiched by two substrates 504,505. The waveguide backlight 500further includes a QWP 506 and a transparent substrate 507 divided intoclear regions 508 and regions supporting half wave retarders 509. In theillustrative embodiment, the grating configuration 501 includes twograting layers 502,503 capable of operating in a different wavelengthband. In many embodiments, the operating wavelength band of the twograting layers covers a large portion of the visible band. Each gratinglayer further includes two interspersed sets of grating elements 510,511and 512,513 for diffracting upward- (510,512) and downward-going(511,513) TIR light. As can readily be appreciated, the backlight shownin FIG. 5 can operate in accordance with principles similar to thoseshown in FIG. 4 . In FIG. 5 , the two grating layers are shown inseparate adjacent layers, the combination of which provides the gratingconfiguration. In other embodiments, the grating elements across the twograting layers are multiplexed and superimposed into a single layer. Forexample, grating elements 510 can be multiplexed with grating elements512, and grating elements A 311 can be multiplexed with grating elements513.

Although FIGS. 4 and 5 illustrate specific polychromatic waveguidebacklight implementations, various configurations can be implemented asappropriate depending on the specific requirements of a givenapplication. For example, in several embodiments, the waveguidebacklight includes two waveguide layers, each containing interspersedgrating elements configured to operate in two wavelength bands. FIG. 6conceptually illustrates a waveguide backlight 600 having two waveguidelayers 601,602 each containing a grating layer 603,604 with alternatingfirst wavelength-diffracting 605 and second wavelength-diffracting 606grating elements in accordance with an embodiment of the invention. Thegrating elements 605,606 can all have K-vectors configured to diffractone of upward-going or downward-going TIR light through an outer surface(e.g., upward-going in the illustrative embodiment of FIG. 6 ) of thewaveguide. In the illustrative embodiment, the firstwavelength-diffracting and second wavelength-diffracting gratingelements 605,606 are spatially overlapped. During operation, firstwavelength P-polarized light 607 and second wavelength P-polarized light608 can be coupled into the waveguides and undergo diffraction andextraction as indicated by rays 609,610 corresponding to firstwavelength light and rays 611,612 corresponding to second wavelengthlight. The waveguide structure of FIG. 6 can be equivalently implementedusing a variety of different grating configurations. Similar to theembodiment shown in FIG. 5 , the backlight shown in FIG. 6 can beimplemented with a single waveguide layer. FIG. 7 conceptuallyillustrates a waveguide backlight having a single waveguide layer withalternating wavelength-diffracting grating elements in accordance withan embodiment of the invention. As shown, the waveguide backlight 700includes a single grating configuration 701 sandwiched by two substrates702,703. The grating configuration 701 includes two grating layers704,705. In the illustrative embodiment, two sets of grating elements706,707 are interspersed within and across both grating layers 704,705.Grating elements from the first set 706 spatially overlap gratingelements from the second set 707. The grating elements can be configuredin a variety of different ways. In some embodiments, each set of gratingelements are configured to diffract a specific wavelength band. In manyembodiments, all of the grating vectors are configured to have similarK-vectors. In the embodiment of FIG. 7 , all of the grating elements areconfigured with diffract and direct light towards the same direction. Ascan readily be appreciated, the waveguide backlight shown in FIG. 7 canoperate in accordance with principles similar to those shown in FIG. 6 .In FIG. 7 , the two grating layers 704,705 are shown in separateadjacent layers, the combination of which provides the single gratingconfiguration 701. In other embodiments, the grating elements within thetwo grating layers 704,705 are multiplexed and superimposed in the samelayer—i.e., each multiplexed region contains grating element 706 andgrating element 707.

Although FIGS. 2-7 illustrate specific waveguide backlights receivingP-polarized input light, waveguide backlights in accordance with variousembodiments of the invention can be configured for operation withvarious light sources. FIG. 8 conceptually illustrates a waveguidebacklight 800 having two waveguide layers 801,802 with alternatingwavelength-diffracting grating elements for input light havingorthogonal polarizations in accordance with an embodiment of theinvention. As shown, the first waveguide layer 801 includes a firstgrating layer 803 with a first set of alternating firstwavelength-diffracting 804 and second wavelength-diffracting 805 gratingelements. Similarly, the second waveguide layer 802 includes a secondgrating layer 806 with a second set of alternating firstwavelength-diffracting 804 and second wavelength-diffracting 805 gratingelements. The waveguide backlight 800 further includes a QWP 807.Various light sources can be implemented as appropriate with suchconfigurations. In the illustrative embodiment, the input light includesfirst and second wavelength light 808,809 having orthogonalpolarizations. For example, the input first wavelength light 808 can beP-polarized and the input second wavelength light 809 can beS-polarized. In the illustrative embodiment, the firstwavelength-diffracting grating elements 804 have K-vectors configured todiffract upward-going TIR light through the upper waveguide surface 810of the first waveguide layer 801, and the second wavelength-diffractinggrating elements 805 have K-vectors configured to diffractdownward-going TIR light through the lower waveguide surface 811 of thesecond waveguide layer 802. As shown, the first wavelength-diffractingand second wavelength-diffracting grating elements 804,805 are spatiallyoverlapped. The light (second wavelength light) extracted from the lowerwaveguide surface 813 has its polarization rotated from S to P by theQWP 807 before being retransmitted through the two waveguide layers801,802 and out of the upper surface 810. Hence, the output light fromthe waveguide backlight 800 is all P-polarized. Similar to theembodiments shown in FIGS. 5 and 7 , the configuration shown in FIG. 8can be implemented within a single waveguide layer. FIG. 9 conceptuallyillustrates a waveguide backlight implementation 900 of the embodimentof FIG. 8 using a single waveguide layer 901 with adjacent gratinglayers 902,903. As can readily be appreciated, such a waveguidebacklight can operate according to principles similar to those shown inFIG. 8 . Furthermore, such grating layers can also be implemented as asingle layer containing multiplexed gratings.

An important principle underlying the embodiments discussed above isthat Bragg gratings diffract with high efficiency when light satisfiesthe Bragg equation to within angular and wavelength tolerances set bythe angular and spectral bandwidths of the grating. The spectral andangular bandwidths can be computed using theory of volume holographicgratings. Waveguided rays falling within the above bandwidth limits arereferred to as being on-Bragg while rays falling outside the bandwidthare referred to as off-Bragg.

Another factor to be addressed in displays and illumination devices,particularly those using lasers, results from beam edge mismatching as abeam undergoes TIR. For a waveguide of thickness D, distance betweensuccessive beam-surface interactions W, and a TIR angle U, the conditionfor seamless matching of upward and downward going TIR beams is given bythe equation 2Dtan(U)=W. When this condition is not met, gaps oroverlaps between adjacent beam portions can occur, which result in anon-uniformity in the output illumination called banding. Banding can bealleviated to some extent by using broadband sources such as LEDs.However, the effect can be much more difficult to overcome with lasers.In many embodiments, the waveguide backlight can be configured tooperate entirely in collimated space. In other words, the input lightand the output beams replicated at each beam grating interaction are allcollimated. In some embodiments, the input beam is scanned in at leastone angular direction. In several embodiments, the cross section of theinput beam can be varied with incidence angle to match a debandingcondition according to the embodiments or teaching disclosed inPCT/US2018/015553 “WAVEGUIDE DEVICE WITH UNIFORM OUTPUT ILLUMINATION”,the disclosure of which is incorporated herein by reference in itsentirety. In a number of embodiments, the input beam cross section canbe adjusted by means of edges formed on a surface or layer supported bythe waveguide as discussed in the above references.

In many embodiments, light is coupled into the waveguide using a gratingor a prism. In many embodiments, the optics for coupling light into thewaveguides may further include, beam splitters, filters, dichroicfilters, polarization components, light integrators, condenser lenses,micro lenses, beam shaping elements and other components commonly usedin display illumination systems.

In many embodiments, the light source is a laser scanned in at least oneangular direction using an electromechanical beam deflector. In someembodiments, the laser scanner may be an electro optical device.

In many embodiments, light can be extracted from the waveguide intooutput paths that are angularly separated. In many embodiments thatoutput paths can be substantially normal to a total internal reflectionsurface of the waveguide. In many embodiments, the light extracted fromthe waveguide is collimated.

In many embodiments, a grating element includes at least one selectedfrom the group of a planar grating, a grating with optical power, agrating providing optical retardation, and a grating with diffusingproperties. In many embodiments, the grating elements can have spatiallyvarying diffraction efficiencies to enable extraction of light along thewaveguide. In many embodiments, the grating elements have diffractionefficiencies proportional to voltages applied across the electrodes. Insome embodiments, the grating elements can have phase retardationsproportional to voltages applied across said electrodes. In manyembodiments, the grating elements can be configured as a one-dimensionalarray of elongate elements. In many embodiments, the gratings can beconfigured as two-dimensional arrays. In many embodiments, the gratingselements are recorded in a Holographic Polymer Dispersed Liquid Crystal.In many embodiments the spatio-temporal addressing of grating elementsby an electrical control circuit addresses can be characterized by acyclic process. In many embodiments, the spatio-temporal addressing ofgrating elements by an electrical control circuit can be characterizedby a random process.

DOCTRINE OF EQUIVALENTS

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof. It istherefore to be understood that the present invention may be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

What is claimed is:
 1. An optical illumination device comprising: alight guiding structure with an upper surface for extractingillumination and a lower surface; a light source optically coupled tosaid light guiding structure and configured to provide polarized light,said light undergoing total internal reflection within said lightguiding structure; and at least one plurality of grating elementsdisposed in at least one grating layer for extracting light from saidlight guiding structure.
 2. The optical illumination device of claim 1,wherein said light source is configured to emit at least first andsecond wavelength collimated light color sequentially, wherein said atleast one plurality of grating elements comprises a first plurality ofgrating elements for diffracting said first wavelength light out of saidlight guiding structure into a first set of output paths, and a secondplurality grating elements for diffracting said second wavelength lightout of said light guiding structure into a second set of output pathssubstantially overlapping said first set of output paths.
 3. The opticalillumination device of claim 2, further comprising a substrate havinghalf-wave retarding regions interspersed with clear regions overlayingsaid upper surface, wherein each said half wave retarding regionoverlaps at least one grating element in each of said first and secondpluralities of grating elements; and wherein each said clear regionoverlaps at least one grating element in each of said first and secondpluralities of grating elements.
 4. The optical illumination device ofclaim 2, further comprising a quarter-wave retarding layer disposed,said quarter-wave retarding layer having a first surface disposed inproximity to said lower surface and a reflective surface.
 5. The opticalillumination device of claim 2, wherein said first plurality of gratingelements is disposed in a separate grating layer to said secondplurality of grating elements, wherein grating elements for diffractingsaid first wavelength light overlap grating elements for diffractingsaid second wavelength light.
 6. The optical illumination device ofclaim 2, wherein grating elements for diffracting first and secondwavelength light are disposed as uniformly interspersed first and secondmultiplicities of grating elements in one layer.
 7. The opticalillumination device of claim 2, wherein grating elements for diffractingfirst and second wavelength light are disposed as uniformly interspersedfirst and second multiplicities of grating elements in two layers,wherein grating element for diffracting a first wavelength light overlapgrating elements for diffracting second wavelength light.
 8. The opticalillumination device of claim 2, wherein grating elements for diffractingfirst wavelength light have a first grating vector and grating elementsfor diffracting second wavelength light have a second grating vector inan opposing direction to said first grating vector.
 9. The opticalillumination device of claim 2, wherein grating elements for diffractingfirst wavelength light and grating elements for diffracting secondwavelength light have grating vectors aligned in substantially paralleldirections.
 10. The optical illumination device of claim 2, whereingrating elements for diffracting first wavelength light and gratingelements for diffracting second wavelength light are off-Bragg withrespect to each other.
 11. The optical illumination device of claim 2,wherein grating elements for diffracting first wavelength light aredisposed in a first layer in which grating elements having a firstgrating vector and grating elements having a second grating vector in anopposing direction to said first grating vector are uniformlyinterspersed, wherein grating elements for diffracting second wavelengthlight are disposed in a second layer in which grating elements having afirst grating vector and grating elements having a second grating vectorin an opposing direction to said first grating vector are interspersed.12. The optical illumination device of claim 2, wherein said firstwavelength light has a first polarization and said second wavelengthlight has a second polarization orthogonal to said first polarization.13. The optical illumination device of claim 2, wherein said firstwavelength light and said second wavelength light have the samepolarization.
 14. The optical illumination device of claim 2, whereingrating elements for diffracting first and second wavelength light aredisposed as first and second multiplicities of grating elementsmultiplexed in a single layer, wherein grating elements for diffractingsaid first wavelength are multiplexed with grating elements fordiffracting said second wavelength light.
 15. The optical illuminationdevice of claim 2, wherein grating elements for diffracting first andsecond wavelength light are disposed as first and second multiplicitiesof grating elements in a stack of two contacting layers with gratingelements for diffracting said first wavelength light overlapping gratingelements for diffracting said second wavelength light.
 16. The opticalillumination device of claim 2, wherein grating elements of said firstplurality are switched into a diffracting state when said light sourceemits said first wavelength light and grating elements of said secondplurality are switched into a diffracting state when said light sourceemits said second wavelength light.
 17. The optical illumination deviceof claim 2, wherein said output paths are angularly separated.
 18. Theoptical illumination device of claim 2, wherein said output paths aresubstantially normal to said upper surface.
 19. The optical illuminationdevice of claim 1, wherein said at least one plurality of gratingelements is disposed in at least one grating layer, wherein said lightguiding structure comprises at least one waveguide, wherein each saidwaveguide supports at least one of said grating layers.
 20. The opticalillumination device of claim 1, wherein said layer is formed betweentransparent substrates with transparent conductive coatings applied toeach said substrate, at least one of said coatings being patterned intoindependently addressable elements overlapping said grating elements,wherein an electrical control circuit operative to apply voltages acrosseach said grating elements is provided.